Revoke a pre-authorized link key

Withdraw a key's authority. Idempotent, and silent about what it did.

204 for a key this call revoked, for one that was already revoked, for an id that does not exist and for one belonging to another account: a status that varied would make this an existence oracle for rows the caller does not own. The audit row is written only when something actually changed, so a double click does not manufacture a second revocation in the trail.

Nodes the key already enrolled are untouched — revoking the key that created a node does not revoke the node.